An artist’s conception of Planet Nine.

An artist’s conception of Planet Nine. Photo: Caltech | R. Hurt (IPAC).

Did you know about Planet Nine? Not to be confused with Pluto, which had its classification changed some years ago to a dwarf planetoid. Planet Nine is a mysterious planet with mass about the same as Neptune and has a wildly elliptical orbit about ten times further from the sun than Pluto.

Scientists only presented evidence of its existence earlier this year; so don’t feel bad if you missed it. It took a long time to discover because it orbits between 40 billion and 140 billion miles away from the sun. By comparison, that’s between 40 and 1,500 astronomical units, and one astronomical unit is the distance between the sun and Earth. So it’s far out there.

It’s so far out there, in fact, that the most interesting question about it right now is why it’s that far away. According to a few new articles by scientists at Harvard, University of Michigan, and the University of Utah, there are several possible explanations, none of which have a huge probability of being true.

It could, for example, have been pulled there by the gravity of a passing star, or our own sun could have stolen it from another solar system. It could have formed closer to the sun but have been propelled to that distance by the combined forces of the gas giants Jupiter and Saturn. Maybe it was a rogue planet flying through space that got caught in the sun’s gravity. That one seems pretty unlikely.

As we learn more about Planet Nine, chances are it will tell us a lot about how planets form and settle into their orbits. Since it’s such an odd orbit to begin with, figuring out how it got there will tell us more about gravitational forces, and maybe the history of our little section of the galaxy.


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